The present invention relates to a laser irradiation apparatus and a method of treating a semiconductor thin film, and particularly to a laser irradiation apparatus used preferably for a treatment for crystallizing a semiconductor thin film, and a method of treating a semiconductor thin film conducted by use of the laser irradiation apparatus.
Thin film transistors (abbreviated to TFT) widely used as switching devices for flat-type display systems such as liquid crystal display systems include the TFT using polycrystalline silicon as an active layer (polycrystalline silicon TFT) and the TFT using amorphous silicon as an active layer (amorphous silicon TFT). Among these, the polycrystalline silicon TFT is higher in driving current as compared with the amorphous silicon TFT, so that it has the merit that it is possible to miniaturize the switching devices and enlarge the numerical aperture of pixel. In addition, the polycrystalline silicon TFT can be used not only as the switching device but also as peripheral driving circuits, for example, a shift register or a level converter, and these peripheral circuits can be formed on a display substrate in the same step as formation of the switching device. For these reasons, the polycrystalline silicon TFT is used as a device for high-precision display systems.
In recent years, a technology for fabricating the polycrystalline silicon TFT by a low-temperature process at or below 600xc2x0 C. (the so-called low temperature polysilicon process) has been developed and put to practical use. By applying the low temperature polysilicon process to the production of a flat-type display system, it becomes unnecessary to use a high heat-resistant but expensive substrate such as quartz and single-crystalline silicon as the display substrate, so that it is possible to achieve reductions in cost and size of the high-precision flat-type display system.
Here, the low temperature polysilicon process is a method of obtaining a polycrystalline silicon layer by irradiating a silicon layer (amorphous silicon layer) formed on a substrate with laser light or electron beam to rapidly heat and melt the silicon without damaging the substrate, and crystallizing the silicon through the subsequent cooling process to obtain the polycrystalline silicon layer.
In order to obtain a polycrystalline silicon layer with a greater grain diameter in such a low temperature polysilicon process, the method of irradiating the silicon layer with the laser light or electron beam is important. In the low temperature polysilicon process at present, a multi-shot irradiation method is widely used. In the multi-shot irradiation method, the laser beam is scanningly radiated onto the silicon layer, and the same portion of the silicon layer is irradiated with laser at least two times, typically, 10 to 20 times. By this, it is known that, for example, in the case of a silicon layer having a thickness of 50 nm, a polycrystalline silicon layer with a grain diameter of 0.1 to 5 xcexcm, typically, about 0.3 to 1 xcexcm is obtained.
Other than the low temperature polysilicon process applying the multi-shot irradiation method mentioned above, there is known a sequential lateral solidification method (hereinafter referred to as SLS method) as reported, for example, in Applied Physics Letters, vol. 69, pp. 2864 to 2866 (1996). FIG. 13 shows an outline of a treatment of a semiconductor thin film by the SLS method. In the method shown in the figure, first, a laser beam H oscillated from a laser light generating means 1 is made to be incident on a mask 6 having a periodic light-dark pattern, by use of optical means 2 to 5 such as lenses and reflectors. The laser beam H transmitted through the mask 6 is radiated onto a silicon layer on the surface of a substrate W mounted on a stage 9 through a focusing lens 7 and a reflector 8, whereby the silicon layer is perfectly melted in a width of several xcexcm. At the time of cooling, crystals are grown laterally from peripheral portions toward the inside of the melting regions, and stripe form lateral growth regions are obtained. Next, the mask 6 or the stage 9 with the substrate W mounted thereon is mechanically moved by a distance not more than the width of the melting regions, typically, about 0.75 xcexcm, and then lateral growth is caused similarly to the above. It is reported that by such a method, it is possible to obtain an elongate polycrystalline silicon thin film which is uniform over a wide area and has grain boundaries parallel to the scanning direction.
Further, as an example of application of the SLS method, Japanese Patent Laid-open No. 2000-150412 discloses a method in which the above-mentioned periodic light-dark pattern is formed as an interference fringe by interference of laser light. The publication also discloses the technique of changing the positions of the interference fringe and, hence, the melting positions of the silicon layer, by moving mirrors and a stepped transmissive plate disposed on the optical path of the laser light by a mechanical means.
However, in the process applying the multi-shot irradiation method, of the above-mentioned low temperature polysilicon processes, the crystal size of the polycrystalline silicon obtained (grain diameter: 0.1 to 5 xcexcm) is extremely small as compared with the size of the thin film transistors at present (about 5 to 50 xcexcm square). Therefore, the characteristic of the thin film transistor fabricated by use of the polycrystalline silicon is, for example, such that the electron mobility is as low as 100 cm2/Vs due to carrier trap at grain boundaries of the polycrystalline silicon; thus, the thin film transistor obtained is inferior to the transistor fabricated in single-crystalline silicon.
Here, in a display system using a thin film transistor, if the characteristics of the thin film transistor in a display area are dispersed, it is recognized as dispersion of display characteristics, resulting in low display quality. The dispersion of the thin film transistor characteristics is due primarily to the dispersion of polycrystalline grain diameter, which arises from dispersion of laser energy in the polycrystallization process, specifically, dispersion on an irradiation shot basis and distribution of light intensity in the irradiation plane.
FIG. 14 shows the variation of mean grain diameter of polycrystalline silicon against laser energy in the case where the same portion is irradiated with laser light 20 times. From the figure, it is seen that where the laser energy may possibly vary by xc2x18%, if the laser energy exceeds the irradiation energy of 380 mJ/cm2 corresponding to the maximum grain diameter even once in the 20 times of irradiation, the grain diameter is rapidly lowered, and fine crystallization occurs partly, so that it is necessary to perform irradiation at 350 mJ/cm2. It is also seen that the dependency of grain diameter on laser energy is heavy and, for example, a dispersion of energy of only xc2x11% leads to a dispersion of grain diameter of no less than about xc2x110%. However, suppressing the dispersion of laser energy to, for example, within xc2x10.5% is difficult to achieve at present, because the pulse oscillation is performed in a short time (for example, in the case of excimer laser, the pulse width is 20 to 200 ns); accordingly, the crystal grain diameter is dispersed.
On the other hand, in the SLS method, it is possible to obtain large-grain-diameter crystals uniform over a wide area. However, since the irradiation of a semiconductor thin film with laser light is conducted through the mask 6 and the lenses 5, 7 of the focusing optical system, utilization efficiency of light energy is lowered, resulting in increases in treatment time and cost per substrate. In addition, there is need for a mechanism for correction of focusing errors due to waviness of the substrate or the like, which also leads to increase in treatment time and cost per substrate. Further, the stage on which to mount the substrate needs a movement accuracy on the order of 1 xcexcm, so that equipment cost is high. Besides, it is difficult to enlarge the irradiation area in view of production cost of the focusing optical system and distortion of image, and, therefore, there is the problem that the treatment takes much time.
In the method of utilizing the interference of laser light as an application of the SLS method, utilization efficiency of light energy is good, but there are the following problems. Namely, in this method, at the time of varying the optical path lengths of split laser light so as to vary the positions of the interference fringe, the mirrors and the stepped transmissive plate are moved by a mechanical means. Here, in the case of using laser light which is oscillated in a pulse form, it is necessary to synchronize the displacement of the interference fringe with the oscillation frequency. However, in the method of displacing the interference fringe by moving the mirrors and the stepped transmissive plate by the mechanical means as above-mentioned, it is impossible to synchronize the movement of the mirrors and the stepped transmissive plate with the laser light oscillated in a pulse form at a high frequency, due to limitations as to the moving velocity and movement accuracy of the mirrors and the stepped transmissive plate. Therefore, it is difficult to perform a treatment by applying pulse-oscillated laser light at a high frequency of more than 10 kHz, such as the laser light oscillated from a pulse oscillation solid laser based on laser diode excitation, and this difficulty hampers an increase in the speed of treatment of a semiconductor thin film.
It is an object of the present invention to provide a laser irradiation apparatus capable of moving at a high frequency a light pattern formed by an interference action of laser light and thereby contriving an increase in the speed of treatment with the laser light, and a method of treating a semiconductor thin film by which it is possible to perform at a high speed a treatment for obtaining large-grain-diameter polycrystalline silicon with excellent uniformity of grain diameter by use of the laser irradiation apparatus.
In order to obtain the above object, in accordance with one aspect of the present invention, there is provided a laser irradiation apparatus, including a laser light generating means, a splitting means for splitting the laser light generated from the laser light generating means into a plurality of beams, a light interference means for causing the beams split by the splitting means to interfere with each other to form a periodic light pattern, and a phase shifting means for electro-optically shifting the phase of at least one of the plurality of beams split by the splitting means.
In the laser irradiation apparatus thus constituted, the laser light generated from the laser light generating means is split by the splitting means into a plurality of beams, which are caused by the light interference means to interfere with each other, thereby forming a periodic light pattern. Therefore, it is possible to generate a light pattern with utilization efficiency of laser light energy maintained at a higher value, as compared with the case of forming a light pattern by use of a mask. In addition, since the phase shifting means for shifting the phase of at least one of the beams split from the laser light is provided, it is possible to optically move the light pattern formed by interference. Besides, since the phase shifting means is for electro-optically shifting the phase, optical movement of the light pattern can be performed at a high frequency. As a result, it is possible to contrive an increase in the speed of treatment. Therefore, it is possible to contrive a reduction in treatment time in treating a semiconductor thin film with laser light. Further, since it is possible to shift the phase at a high frequency synchronized with the laser light generating means for pulse oscillation at a high frequency, it is possible to use a laser light generating means in which the life of an excitation light source is long but the repeated pulse oscillation frequency is high, such as, for example, a pulse oscillation solid laser based on laser diode excitation. Therefore, in the laser irradiation apparatus for performing a treatment by optically moving the light pattern, it is possible to build up an apparatus which can operate continuously for a long time and which is high in reliability.
In accordance with another aspect of the present invention, there is provided a method of treating a semiconductor thin film including the steps of irradiating the semiconductor thin film with a periodic light pattern generated by interference of a plurality of beams split from laser light, thereby partially melting the semiconductor thin film, and thereafter moving the light pattern in the arrangement direction of the light pattern within the range of the period thereof, wherein the movement of the light pattern is carried out by electro-optically shifting the phase of at least one of the split beams.
In the treating method, at the time of shifting the phase of the beam for optically moving the light pattern formed by interference of the split beams, so as thereby to obtain large-grain-diameter polycrystalline silicon with excellent uniformity of grain diameter, the phase of the beam is shifted electro-optically, so that the optical movement of the light pattern is performed at a high frequency. Therefore, a treatment by moving the light pattern at a high speed is conducted, and the speed of treatment is enhanced. Therefore, the treatment time in treating the semiconductor thin film for obtaining large-grain-diameter polycrystalline silicon with excellent uniformity of grain diameter can be shortened.